Electric measuring instrument.



PATENTED NOV. 6, 1906.

W. H. PRATT. ELECTRIC MEASURING INSTRUMENT.

APPLICATION FILED MAY24,1904.

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Inventor",

I l'iam H. Pratt,

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PATENTED NOV. 6, 1906.

W. H. PRATT. ELECTRIC MEASURING INSTRUMENT.

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II l T I II APPLIOATION FILED MAYM, 1904.

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WILLIAM H. PRATT, OF LYNN, MASSACHUSETTS, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

ELECTREO MEASURENG i1NSTRUWEENTn Specification of Letters Patent.

Patented. Nov. 6, 1906.

Application filed May 24, 1904. Serial No. 209,571.

To all whom it may concern:

Be it known that I, WILLIAM H. PRATT, a citizen of the United States, residing atLynn, county of Essex, State of Massachusetts, have invented certain new and useful Improvements in Electric Measuring Instruments, of which the following is a specification. i

This invention relates to electric indicating instruments; and its object is to effect certain improvements in instruments ofthis type whereby the movements of the moving indicating system per unit of the energy or energy factor measured by the instrument shall be equal at all parts of a wide range of movement of the indicator, so that the mark ings on thescale of the instrument shall be evenly distributed throughout the scale.

A further object of the invention is to simplify the construction of such instruments, and thereby reduce the cost of production.

Electric indicating instruments have been constructed heretofore having a countertorque device consisting. of a return element of magnetizable material mounted for rota tion in a magnetic field to furnish the zeroseeking force. This return-piece has been mounted obliquely on the shaft of the instrument, and the pole-faces of the magnet have been chamferedoff, so that when the indicator is at zero the axis of the return-piece is substantially perpendicular to the pole-faces, as is shown and described in the. patent to Robinson, No. 751,015. With such a construction the movements of the indicator per unit of the energy or energy factor measured are not exactly equal at all partsof the scale, so that the scale-markings are not as evenly distributed as is desired, the markings being more open at the u per end of the scale than at the lower. ter much experiment I have found that by varying the shape of the pole-faces which are presented to the return element the scale-markings can be made even throughout the scale or can be made open at any desired portion and closed at another or other portions.

When it is desired to make the scale-markings even throughout, I so form the polefaces cooperating with the return element that they overhang the latter somewhat and partially surround it, so that the air-gap between the pole-faces and the-return-piece as the latter turns with the shaft of the instru ment either increases very slightly or does not increase at all. In thelatter case, and preferably in all cases, I use a return-piece having its ends rounded off not only in order to secure a smooth counter-torque gradient, but also for the'reason that though the actual shortest distance between the return-piece and the pole-faces does not increase the airgap for all the effective lines of force which thread the return-piece increases slightly, so that the return-piece always exerts a force tending to bring the indicator to the zero position.

For certain uses of electric indicating instruments permanent magnets are preferred to electromagnets; but on account of the hardness and brittleness of the steel from which permanent magnets are made forming pole-faces of special shape on them or securing separate pole-pieces to them has hereto fore been both difficult and expensive I have therefore provided pole-pieces for the magnets which fit on a non-magnetic frame and are held tightly thereon by the spring action of the legs of the magnets, and I find that this arrangement of the parts simplifies the instrument and effects a great saving in the cost of construction. V

The novel features of my invention will be definitely indicated in the claims appended hereto. The details of construction and the mode of operation of my improved electric indicating instrument will be better understood by reference tothe following description, taken in connection with'the accompanying drawings, which show the preferred embodiment of my invention.

In the drawings, Figure 1 is a top view of the instrument with the casing in section and the scale removed. Fig. 2 is a sectional elevation of the same. Fig. 3 is a perspective view of two of the pole-pieces and the shaft of the instrument.

the magnetic return-piece. F i 5 is a per- Fig. 4 is an elevation of spective view of the armature roken away in part, and Fig. 6 is a perspective View of a modified form of return-piece.

Referring to the drawings, 1 indicates a base, on whichis mounted a frame 2, of castbrass or other non-magnetic material, having arms in which the magnets 3 3 are held, the

magnets having their poles oppositely arranged in order to form an astatic field. At the lower end frame 2 is'extended to form curved jaws 4 4, and the pole-pieces 5 5 are rounded on one side to fit within these curved jaws. On one end each pole-piece 5 is provided with a flange 6, which overhangs the side of one of the curved jaws 4 4 to support the pole-piece in position. Pins 7 7 are provided in the jaws 4 4 to properly locate the pole-pieces, and the latter are accurately machined, so that they fall readily into the proper-position and fit tightly therein. The adjacent surfaces of the legs of magnets 3 3 are ground true, and the magnets are of such size that by springing the legs apart slightly they can lie-made to embrace the pole-pieces 5 5 when the latter are supported in position on the jaws 4 4 of frame 2, which serve as spacers to position the pole-pieces the requisite distance apart. The pole-pieces are thus held firmly in the proper position and in hard contact with the legs of the ma nets by the spring action of the magnets t remselves, thus obviating the use of screws or clam s for securing them to the magnets and avoiding a large part of the expense and trouble usually encountered in providing permanent magnets with pole-pieces of special shape.

Suitably mounted on base 1 is an auxiliary frame 8, in which are bearings for the shaft 9 of theinstrument, which carries the armature 10, the latter being arranged for rotation between the poles 5 5 of each of the magnets. The construction of the armature is best shown in Fig. 5. lie disk 11, serving as a damper forthe oscillations of the indicator, as is well understood, on either side of which the coils 12 are mounted, as shown, and covered with sheet aluminium 13 or other suitable material. Current is led to the armature-coils by the ductile spirals 14, each of which has one end attached to the shaft 9 and the other to a bracket 15, secured to but insulated from frame 8. Shaft 9 carries an indicator 20, which moves back and forth over a scale 21, suitably mounted on the base ofthe instrument, and the casing 22, which incloses the parts, is provided wit 1 a glass, through which the positions of the indicator with reference to the scale are read.

In addition to the faces presented to the armature 10 pole-pieces 5 5 have auxiliary pole-faces formed thereon to establish magnetic fields between the opposite poles of the two magnets. Mounted on snaft 9, at an incline thereto, are the magnetic return-pieces 16 16, adapted to rotate in these auxiliary magnetic fields to furnish a counter torque opposing the turning moment and tending to restore the moving indicating system to the Zero position. The magnetic returnpiere which I prefer to'use is that shown in Fig. 4,

It consists of-a metal and consists of a rod 16 of such small section that it is at all times saturated by the flux of the magnets, having its ends enlarged and rounded off, as shown at 17.

The form of return element shown in Fig. 6 is a body 18 of non-magnetic material having strips of filaments 19 of iron, nickel, or other magnetic substance on its sides, the purpose of the longitudinal divisions being to prevent the fiuX from flowing across the piece. The auxiliary pole-faces of the magnets are concaved or hollowed out, so that each polepiece hasa portion overhanging the returnpiece 1 6, as shown. This 0 verhanging portion carries the lines of force as the return-piece turns with shaft 9, so that the air-gap increases in length very slightly or not at all. If the overhanging portion of the pole-piece is so formed that the actual shortest distance traversed by any of the lines of force in passing from the pole-piece to the return-piece does not increase, still the air-gap for all the effective lines of force threading the return-piece will increase somewhat, and that increase will be smooth and gradual, since the area of the enlarged rounded ends of the returnpiece that is within this minimum distance of the pole-pieces is gradually reduced as the rounded ends recede from the portions of the pole-faces which bend aroundmore nearly parallel to shaft 9. By this construction, employing a return-piece to furnish counter torque, mounted on the shaft of the instrument at an incline thereto in a magnetic field, the axis of which is also inclined to the shaft, the range of indication of the instrument ex tends through one hundred and twenty degrees or more, and I-find'that by shaping the pole-faces as shown herein the movements of the moving indicating system per unit of the energy or energy factor measured are equal at all parts of the scale, so that the scale markings are evenly distributed for the whole of this wide range of indication.

What I claim as new, and desire to secure by Letters Patent of the United States, is-

1. An electric measuring instrument having a rotatable shaft, magnets for establishing a magnetic field, a magnetic return element mounted on the shaft at an incline thereto for rotation in said field, and pole pieces for said magnets having concave faces.

2. An electric measuring instrument having a rotatable shaft, magnets for establishing a magnetic field, and a magnetic return element mounted on the shaft at an incline thereto for rotation in said field and furnishing counter torque for the moving indicating system, said magnets having their pole-faces so formed as to partially surround the return element.

3. An electric measuring instrument having means for establishing a magnetic field and a magnetic return element mounted for movement in said field and furnishing counter torque for the moving indicating system, the pole-pieces for said magnetic field being so ormed that the shortest distance between them andsaid element remains constant as the element moves in the magnetic field and the return element being shaped to vary the flux which threads it.

4. An electric measuring instrument having a shaft, and a magnetic return element furnishing counter torque for the moving indicating system mounted on the-shaft and consisting of a rod of magnetizable material having enlarged rounded ends.

5. An electric measuring instrument having a rot at able shaft, a magnetic return-piece to furnish counter torque mounted on the shaft at an incline thereto and consisting of a rod of magnetizable material having enlarged rounded ends, and means for establishing a magnetic field in which the returnpiece revolves, said rod being of such cross-- section that it is always saturated by the flux ofsaid field.

6. In an instrument, a permanent magnet, and pole-pieces therefor clamped in position by the spring action of the magnet.

7. In an instrument, a permanent magnet, a spacer of'non-magnetic material, and polepieces for the magnet clamped in position against the spacer by the spring action of the magnet.

8. In an instrument, a permanent magnet, a spacer of non-magnetic material, and polepieces for the magnet having flanges thereon engaging the spacer, said pole-pieces being clamped to the spacer, by the spring action of the magnet.

9. An electric measuring instrument having abase, a frame of non-magnetic material secured thereto, permanent magnets supported by the frame, extensions on the frame forming jaws, and pole-pieces for the magnets clamped in position against the jaws by the spring action of the magnet.

10. An electric measuring instrument having a base, a frame of non-magnetic material secured thereto, permanent magnets sup ported by the frame, extensions integral with the frame forming spacers, and pole-pieces for the magnets having flanges thereon engaging the spacers, said pole-pieces being clamped to the spacers by the spring action of the ma nets.

In w1tness whereof I have hereunto set my hand this 21st day of May, 1904.

'WILLIAM H. PRATT.

Witnesses:

DUGALD MoK. MoK1LLoi JOHN J. WALKER. 

